1. Field of the Invention
The present invention relates to systems and methods for controlling motors.
2. Related Art
In the prior art, an electric vehicle employs a motor such as a brushless motor including a permanent magnet rotor and a stator having coils to which U, V and W phases of electric current are fed to drive the motor.
A motor control circuit or unit receives a current command value from a vehicle control circuit which controls the electric vehicle. The motor control circuit generates pulse width modulated signals forming the U, V and W individual phases in accordance with the current command value. The pulse width modulated signals are output to a drive circuit.
This drive circuit generates transistor drive signals corresponding to the pulse width modulated signals and outputs the transistor drive signals to an inverter bridge. The inverter bridge has six transistors which are turned ON only when the corresponding transistor drive signals are ON to generate and feed the currents to individual phases of the stator coils. Thus the electric vehicle is run by activating the motor drive unit to drive the motor.
When the values of currents of two of the three phases are determined, the value of the current of the third phase can be computed. In order to control the currents of the individual phases, only the currents of the U and V phases, for example, need be detected by current sensors. Feedback control is performed on a d-q axis model in which the d-axis is taken in a direction of a magnetic pole pair of the rotor and the q-axis is taken in a direction perpendicular to the d-axis.
In the motor control circuit, the sensed currents of the U and V phases are converted into a d-axis current and a q-axis current by a three-phase to two-phase conversion. A deviation of the d-axis current from a d-axis current command value is computed so that a d-axis voltage command value is generated on the basis of the d-axis current deviation. Similarly, a deviation of the q-axis current from a q-axis current command value is computed so that a q-axis voltage command value is generated on the basis of the q-axis current deviation. These d-axis and q-axis voltage command values are converted into U, V and W phase voltage command values by two-phase to three-phase conversion. The pulse width modulation signals of the individual phases are generated from the corresponding voltage command values.
In order to compute the d-axis and q-axis voltage command values using the respective d-axis and q-axis current deviations, corresponding inductances of the motor are employed so as to prevent interference between the d-axis and q-axis voltage command values. These inductances are estimated from the transient changes in the corresponding d-axis and q-axis currents.
However the prior art drive unit estimating the inductances from the transient changes in the d-axis and q-axis currents required complicated estimating computations enlarging the scale and cost of the motor control circuit. Additionally to avoid instability in the currents of the individual phases, relatively long time constants for measuring transient changes are required and such long time constants reduce the quickness of response to changing conditions.